This invention relates, firstly, to a method of eliminating the side tone of a telephone station, more particularly in electronic time-division multiplex telephone exchanges. Secondly, it relates to apparatus for performing this method.
The known means of eliminating the side tone of a telephone station, i.e. in two-wire/four-wire changeover systems, are divided up into those using a telephone transformer and those in which the transmitted signals are separated from the received signals by using, for example, differential amplifiers to subtract the signal present at any given moment on the line and the received signals.
The disadvantage of these means is that they are bulky or expensive and they cannot operate independently of the frequency of the transmitting signals and of the impedance of the telephone line.